In hot working of a metallic material, e.g., steel, spontaneous emission of heat from the hot metallic material occurs even during the working, resulting in cooling-down of the metallic material. This increases the deformation stress in the metallic material and make the working gradually more difficult. At this point, therefore, it is necessary to reheat the material.
Since the loss of energy during repeated heating and working is large, it is desirable from a standpoint of envergy-saving to prevent the emission of heat during the working so as to increase the quantity of work that can be done when heating only once.
In the case of material which is transformed below a temperature at which a hot working is carried out, such as austenite, as the number of repeated heatings is increased, grain growth will occur, leading to a reduction in the quality of the metal. It is, therefore, also necessary from a standpoint of insurance of quality to minimize the number of heating repetitions by preventing the emission of heat during hot working.
Moreover, when the cooling rate is high, the reduction in temperature at the edges of the metallic material proceeds more rapidly than at other areas. This leads to a rapid reduction in ductility and, therefore, to ready formation of defects such as cracks and flaws during the working.
Various attempts have heretofore been made to control the emission of heat during hot working, including a method in which an insulating cover is provided as a part of the equipment for use in hot working and a method in which an insulating cover is provided directly on a hot metallic material to be worked (this method is described in Japanese patent application Laid-Open No. 94745/1980). The insulating cover used in the former method, however, has only a poor insulating effect. The latter method suffers from the disadvantages that it is necessary to prepare a number of different insulating covers depending on the shape of the metallic material to be worked, and that these insulating covers interfere with the hot working of the material.